Heterogeneous networks (HetNet) have been added to the scope of the LTE-A work item. Now enhanced inter-cell interference coordination (eICIC) for co-channel HetNet deployment is one of the key technical points for Release 10.
Co-channel HetNets comprise macrocells and picocells operating on the same frequency channel. Such deployments present some specific interference scenarios for which new Inter-Cell Interference Cancelation (ICIC) techniques are required.
In one scenario, the picocells are open to users of the macrocellular network. In order to ensure that such picocells carry a useful share of the total traffic load, a user equipment (UE) may be programmed to access preferentially to the picocells rather than the macrocells. For example by biasing the Signal-to-Interference plus Noise Ratio (SINR) threshold, the UE may select a picocell to access to. Under such conditions, UEs near the edge of a picocell's coverage area and accessing the picocell will suffer strong interference from one or more macrocells. In order to alleviate such interference, some subframes may be configured as “blank” or “almost blank” in a macrocell. A blank subframe contains no transmission from the macrocell, while an “almost blank” subframe typically contains no data transmission and little or no control signalling transmission, but will contain Reference Signal (RS) transmissions in order to ensure backward compatibility with legacy terminals. The legacy terminals expect to find the reference signals for measurements but are unaware of the configuration of almost blank subframes. Almost blank subframes may also contain synchronization signals, broadcast control information and/or paging signals.
In order to make the use of blank or almost blank subframes (ABSs) effective, signalling is needed from the macrocell to the picocell across the corresponding backhaul interface, known in LTE as the “X2” interface. Note that hereafter the term “ABS” is used, and should be understood to include both blank and almost blank subframes. For LTE Release 10, it has been agreed that this X2 signalling will take the form of a coordination bitmap to indicate the ABS pattern. For example, each bit corresponds to one subframe in a series of subframes, and the value of the bit indicates whether the subframe is an ABS or not. For example, 1 indicates the subframe is an ABS, while 0 indicates the subframe is not an ABS, or vice versa. Such signalling can help the picocell to schedule data transmissions in the picocell appropriately to avoid interference, e.g. by scheduling transmissions to UEs near the edge of the picocell during ABSs, and to signal to the UEs the subframes which have low macrocellular interference and may therefore be used for RRM (Radio Resource Management)/RLM (Radio Link Monitoring)/CSI (Channel State Information) measurements.
However, as mentioned before, in order to ensure backward compatibility with legacy terminals, the cell reference signals (CRS) for measurements and some basic physical channels, such as synchronization signals (SSS (Secondary Synchronization Signal)/PSS (Primary Synchronization Signal)), broadcast control information (PBCH (Physical Broadcast Channel, SIB-1 (System Information Block)) and/or paging signals, are still transmitted no matter whether this subframe is configured to be ABS or not. That means the interference caused by Macrocell Base Station (MeNB) CRS on picocells data and control channel in ABS corresponding subframes, as well as the CRS collision and interference on basic physical channels between Macrocell and picocells, always exists. During 3GPP RAN1 discussion, the cell range expansion (CRE) by bias cell selection is considered as one major approach to extending the coverage of the picocell and then balancing the traffic load. When the selected bias value for CRE becomes larger, these two kinds of interference will become more seriously. It is because when the selected bias value for CRE becomes larger, the coverage range of the picocell becomes larger, and the interference from the macrocell strengths as well.
Currently in RAN plenary meeting #51, the “Further Enhanced Non-CA Based ICIC for LTE”, which focuses on the UE performance enhancement, has been agreed as the new work item for Rel-11 eICIC topic. To significantly improve UE DL control and data channel detection ability, it will become the major research major regarding whether to improve UE measurement/reporting mode (including necessary signaling design) on which existing FDD and TDD systems depend and whether to improve the UE performance requirements (achieving related or standard-related improvement) in the presence of dominant interferers (including colliding and non-colliding RS, as well as, MBSFN used as ABS, as well as ABS subframe configurations).
However, until now there is no detailed technical solution to describe how UE enhances its performance requirements and detection ability.